Longitude

Longitude is the angular distance of a location east or west of a chosen prime meridian. Together with latitude, it forms the basis of the geographic coordinate system used to describe positions on Earth. By international agreement, the prime meridian passes through Greenwich in London and is assigned a value of 0 degrees. Longitudes extend eastward and westward from that line to 180 degrees. A location can therefore be described as lying east longitude or west longitude, or by signed decimal values in which east is positive and west is negative.

Lines of equal longitude are called meridians. Unlike lines of latitude, meridians are not parallel to one another. They converge at the poles, where all longitudes meet. At the Equator, the distance represented by one degree of longitude is greatest. That distance decreases steadily toward the poles because the circles of latitude become smaller as they approach the ends of the Earth's rotational axis.

Historically, longitude was far more difficult to determine than latitude. On land, astronomers could compare local observations to known reference times, but at sea the problem remained unsolved for centuries. Accurate determination of longitude depended on precise timekeeping, because longitude is directly related to the Earth's rotation. Since the Earth rotates 360 degrees in 24 hours, it turns 15 degrees each hour. If a navigator knows the local solar time and the time at the prime meridian, the difference can be converted into longitude. This link between time and position made the development of reliable marine chronometers one of the most important technical advances in navigation.

Longitude has many uses beyond navigation. It is fundamental in cartography, surveying, satellite positioning, telecommunications, and geospatial databases. It also underlies the division of the world into time zones, although legal and political boundaries often modify the ideal geometric arrangement.

In practical use, longitude is written in degrees, minutes, and seconds or in decimal degrees. A value such as 118.2437 degrees west places a point west of Greenwich by a little more than 118 degrees. Longitude alone identifies a north south line on the globe. When paired with latitude, it locates a precise point. Modern mapping systems, navigation devices, and scientific instruments rely on longitude as an essential geographic reference.